Many optoelectronic sensors work in accordance with the scanning principle in which a light beam is transmitted into the monitored zone and the light beam reflected by objects is received again in order then to electronically evaluate the received signal. The time of flight (TOF) is in this respect often measured using a known phase method or pulse method to determine the distance of a scanned object. In a pulse averaging process known, for example, from EP 2 469 296 B1, a plurality of individual pulses are transmitted for a measurement and the received pulses are statistically evaluated. Another possibility for determining distances is triangulation. A light transmitter and a spatially resolving light receiver are here arranged next to one another and the position of the received light spot is evaluated in dependence on the object distance.
To expand the measured zone, the scanning beam can be moved, as is the case in a laser scanner. A light beam generated by a laser there periodically sweeps over the monitored zone with the help of a deflection unit. In addition to the measured distance information, a conclusion is drawn on the angular location of the object from the angular position of the deflection unit and the site of an object in the monitored zone is thus detected in two-dimensional polar coordinates. The scanning movement is achieved by a rotating mirror in many laser scanners. It is, however, also known to instead have the total measurement head with light transmitters and light receivers rotate, such as is described in DE 197 57 849 B4.
LEDs or laser diodes serve as the light source of an optoelectronic sensor. Although only one light beam and thus a single light source is needed with a light scanner, it is also known to use a light beam from an array having a plurality of light sources. One reason can be the desire for a higher optical output power.
In particular VCSELs (vertical cavity surface emitting lasers) frequently no longer have a single light emission surface (“mesa), but rather a plurality of light emission surfaces arranged in direct proximity. The reason is that the emission angle of light from small light emission surfaces can be monitored better from the manufacturer's side.
The individual light emission surface is then admittedly smaller, but a perimeter comprising all the light emission surfaces becomes larger. If the light of such a VCSEL is imaged using a single transmission lens, disadvantageous larger light spots result therefrom in the monitored zone and ultimately on the reception element of the sensor.
This problem cannot be solved by a simple additional lens. FIG. 12 shows an exemplary light transmitter 100 having two individual light sources 102a-b and an associated transmission optics 104. An additional individual lens 106 is arranged between the light transmitter 100 and the transmission optics 104. It generates a smaller real intermediate image 108a-b of the individual light sources 102a-b so that first, as desired, the perimeter about the individual light sources 102a-b has become smaller. Since, however, the etendue is a conserved quantity, the smaller intermediate image 108a-b must necessarily be accompanied by an increased angle of radiation ∝2>∝1. No advantage is therefore achieved overall since all the light is no longer incident on the common transmission optics 104.
A further conceivable approach is to associate a microlens field with the VCSEL that has an at least partly separate light path for each light emission surface. A real intermediate image of an associated light emission surface is in particular generated by a respective once microlens.
FIG. 13 shows a first example. The two individual light sources 102a-b of the light transmitter 100 here each have microlenses 106a-b associated with them. The common transmission optics 104 arranged downstream is no longer shown here. Each individual light source 102a-b is imaged onto the same intermediate image 108a-b by the associated microlens 106a-b, with the overlap not having to be perfect and with the common intermediate image 108a-b optionally being larger than an individual light source 102a-b. It can admittedly be achieved in this respect that the angle of divergence of the individual light sources remains constant, but the angle of divergence of the total light beam is further enlarged by the tilting of the main beams. An additional disadvantage of the approach using real intermediate images can be found in the required high refractive power of the microlenses that results in aberrations and thus in enlarged, blurred real intermediate images due to a large curvature of the lens surfaces.
FIG. 14 shows a further example. Unlike in FIG. 13, the intermediate images 108a-b are here separate and are not disposed above one another, but preferably in very close proximity. The disadvantage of the enlarged angle of divergence remains.
The approach using microlenses only works more or less well for as long as the individual light sources 102a-b are comparatively small and are far apart from one another. In many cases, however, the light emission surfaces are large in comparison with their mutual distances in a VCSEL. The microlenses 106a-b now have to approach very close to the respective emission surface so that they are selectively only illuminated by their light. However, from such a small distance, the light emission surface has a large spatial angular expansion. The microlens 106a-b therefore has to cover a respective large image field. An individual lens is only suitable for this purpose with limitations; the images therefore become considerably blurred so that the desired effect of a decrease in size is lost again.
DE 101 56 282 A2 discloses a laser scanner that uses a laser matrix as a light source. Its individual lasers, however, each transmit their own scanning beam to obtain a distance image having so many individual distance points as the laser matrix has individual lasers.
DE 10 2011 052 802 A1 discloses a 3D camera having an illumination unit that has a plurality of individual emitters having respective microoptics in the form of prisms and lenses. A structured illumination pattern is thus generated, wherein a respective individual emitter contributes a light spot that is projected onto a specific location by means of the associated microoptics to obtain a pseudo-random pattern overall. However, it is not the aim here to scan an object with a small light spot, but conversely to illuminate the total detection zone.
A further optical system is known from WO 2014/102341 A1 that generates a structured light pattern, wherein the combination of microprisms and microlenses can again be used.
US 2013/0266326 A1 discloses microlenses for a multibeam VSCEL array. The problem discussed with reference to FIGS. 13 and 14 is, however, not discussed or even solved here.
It is therefore the object of the invention to improve a scanning sensor of the category having a light transmitter composed of a plurality of individual light sources.